In general, an electric power steering apparatus which energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (a steering assist torque), such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a motor current becomes small or zero, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
Here, a general configuration of an electric power steering apparatus will be described with reference to FIG. 1. A column shaft 2 connected to a steering wheel (handle) 1 is connected to tie rods 6 of steered wheels through reduction gears 3, an intermediate shaft 4 including universal joints 4a and 4b, and a rack and pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 14, and an ignition signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value I of an assist command based on a torque signal (a steering torque) Tr detected by the torque sensor 10 and a velocity signal Vel detected by a velocity sensor 12, and controls the motor 20 based on the calculated current command value I through a motor driving unit 21. A voltage Vm applied between motor terminals of the motor 20 and the motor current i are measured and inputted into the control unit 100.
In such the electric power steering apparatus, heretofore, for example as disclosed in Japanese Patent Application Laid-open No. H8-290778 (Patent Document 1), by means of a robust stabilizing compensation section within the control unit 100, stability of the system and sensitivity characteristics of a road information and a disturbance information are simultaneously designed.
However, in such the conventional control device, since a reaction force during steering in the vicinity of a steering neutral point is small, it is difficult to accurately transmit the road information to a driver due to the effect of friction. Further, in the conventional electric power steering apparatus, it is difficult to set a hysteresis characteristic between a steering angle and a steering force to a characteristic at the same level as a hydraulic power steering apparatus.
As an apparatus to solve such a problem, there is an apparatus disclosed in Japanese Patent Application Laid-open No. 2002-369565 (Patent Document 2).
The outline of the apparatus disclosed in Patent Document 2 will be described with reference to FIG. 2 corresponding to FIG. 1. The motor 20 for generating an assist torque of a steering apparatus is driven by a motor driving unit 21, the motor driving unit 21 is controlled by a current command value I from the control unit 100 indicated by a dashed-two dotted line, and the torque signal Tr from the torque sensor and the velocity signal Vel from a velocity detecting system are inputted into the control unit 100.
The control unit 100 comprises a torque system control section 110 indicated by a dashed line that performs a control by using the torque signal Tr and a motor system control section 120 indicated by a dashed-dotted line that performs a control relating to driving of the motor 20. The torque system control section 110 comprises an assist amount calculating section 111, a differential control section 112, a yaw rate convergence control section 113, a robust stabilizing compensation section 114 and a self aligning torque (SAT) estimating feedback section 115A, and further includes adding sections 116A, 116B and a subtracting section 116C. Further, the motor system control section 120 comprises a compensation section 121, a disturbance estimating section 122, a motor angular speed estimating section 123, a motor angular acceleration estimating section (a differential section) 124 and a motor characteristic compensation section 125, and further includes adding sections 126A, 126B.
The torque signal Tr is inputted into the assist amount calculating section 111, the differential control section 112, the yaw rate convergence control section 113 and the SAT estimating feedback section 115A, all of them input the velocity signal Vel as a parameter. The assist amount calculating section 111 calculates an assist torque amount based on the torque signal Tr. The yaw rate convergence control section 113 inputs the torque signal Tr and a motor angular speed ω and brakes a movement that the steering wheel whirls in order to improve the convergence of yaw of the vehicle. Further, the differential control section 112 enhances a responsibility of the control in the vicinity of a steering neutral point and realizes a smooth steering. Moreover, the SAT estimating feedback section 115A inputs the torque signal Tr, a signal obtained in the adding section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, the motor angular speed ω estimated by the motor angular speed estimating section 123 and an angular acceleration *ω from the motor angular acceleration estimating section 124, estimates an SAT as a reaction force from the road, performs a signal processing for the estimated SAT by using a feedback filter, and provides the steering wheel with a suitable road information as the reaction force.
Further, a steering assist command value that is obtained in the adding section 116B by adding the output of the yaw rate convergence control section 113 to the steering assist command value obtained in the adding section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, is inputted into the robust stabilizing compensation section 114 as an assist amount AQ. The robust stabilizing compensation section 114 is a section such as a compensation section disclosed in Japanese Patent Application Laid-open No. H8-290778, removes a peak value in a resonance frequency of a resonance system comprising of an inertia element and a spring element included in the detected torque, and compensates a phase shift of the resonance frequency that obstructs the responsibility and the stability of the control system. By subtracting the output of the SAT estimating feedback section 115A from the output of the robust stabilizing compensation section 114 in the subtracting section 116C, a current command value (the assist amount) Ia that is capable of transmitting the road information to the steering wheel as the reaction force, is obtained.
Moreover, the motor angular speed estimating section 123 estimates the motor angular speed ω based on the voltage Vm applied between the motor terminals and the motor current i, and the motor angular speed ω is inputted into the motor angular acceleration estimating section 124, the yaw rate convergence control section 113 and the SAT estimating feedback section 115A. The motor angular acceleration estimating section 124 estimates the motor angular acceleration based on the motor angular speed ω that is inputted, and the estimated motor angular acceleration *ω is inputted into the motor characteristic compensation section 125. In the adding section 126A, the current command value Ia that is obtained by subtracting the output of the SAT estimating feedback section 115A from the output of the robust stabilizing compensation section 114, is added to the output Ic of the motor characteristic compensation section 125, the added signal is inputted into the compensation section 121 comprising a differential compensator as a current command value Ir. A current command value I that is obtained by adding the output of the disturbance estimating section 122 in the adding section 126B to a current command value Ira compensated by the compensation section 121, is inputted into the motor driving unit 21 and the disturbance estimating section 122. The disturbance estimating section 122 is such an apparatus disclosed in Japanese Patent Application Laid-open No. H8-310417, is capable of maintaining a desired motor control characteristic in an output reference of the control system based on a signal obtained by adding the output of the disturbance estimating section 122 to the current command value Ira compensated by the compensation section 121 that is the control target of the motor output and the motor current i, and does not lose the stability of the control system.
Here, aspects of torques generated between a road and a steering will be described with reference to FIG. 3. When a driver steers the steering wheel 1, a steering torque Th is generated and then the motor 20 generates an assist torque Tm in accordance with the steering torque (the torque signal) Th. As a result, wheels are steered, and an SAT is generated as the reaction force. Further, in such case, due to an inertia J and a friction (a static friction) Fr of the motor 20, a torque that will become the resistance of steering the steering wheel, is generated. By considering a balance between these forces, a motion equation such as the following Expression 1 can be obtained.J·*ω+Fr·sign(ω)+SAT=Tm+Th  [Expression 1]
Here, when setting initial values to zero, performing a Laplace transform (s: Laplace operator) for the above Expression 1 and then solving the SAT, the following Expression 2 can be obtained.SAT(s)=Tm(s)+Th(s)−J·*ω(s)−Fr·sign(ω(s))  [Expression 2]
It is clear from the above Expression 2 that by preliminarily obtaining the inertia J and the static friction Fr of the motor 20 as constants, it is possible to estimate the SAT based on the motor rotation angular speed ω, the rotation angular acceleration *ω, the assist torque Tm and the steering torque Th. From such a reason, the torque signal Tr, the angular speed ω, the angular acceleration *ω and the output of the assist amount calculating section 111 are inputted into the SAT estimating feedback section 115A.
Further, in the case of feeding back the SAT information estimated by the SAT estimating feedback section 115A without any processing, since the steering becomes too heavy, it is impossible to improve the steering feeling. Therefore, as shown in FIG. 4, by using a feedback filter 115AF having a velocity-sensitive gain and a frequency characteristic, a signal processing of an estimate value of the SAT is performed and only necessary and sufficient information for improving the steering feeling is fed back. The feedback filter 115AF used in here includes a Q-filter (phase-lag) 115B having a gain that reduces the amplitude of the estimated SAT to a necessary and sufficient value and a gain section 115C having a gain characteristic that is sensitive about the velocity signal Vel as shown in FIG. 5, and in the case that the importance of road information such as static steering and low speed driving is relatively low, decreases the road information to feed back.
Although the apparatus described in the above Patent Document 2 configures the feedback of SAT-estimating so that a frequency band in which there are disturbances needed to suppress and a frequency band in which there are disturbances needed to transmit are compatible, there is no function that actively cancel out disturbances needed to suppress.
On the other hand, in the vehicles, at an ordinary braking and a steady-state running, brake judder and shimmy that give annoyance to passengers occur. The brake judder is a floor and pedal vibration occurring at braking of the vehicle, and sometimes induces a steering rotational vibration. The excitation source of the brake judder is a variation in the braking torque due to DTV (Disk Thickness Variation) of the brake disk, and it has the first and higher order contents of the wheel rotation. The brake judder is amplified by the resonance or the like at front and rear of the suspension, transmitted through the vehicle body and the steering system, and ultimately becomes the floor and pedal vibration and the steering vibration. Further, the shimmy is a vibration in steering rotation direction during the vehicle running. The excitation source is an imbalance and non-uniformity of rotating parts such as tires and wheels. The shimmy is amplified by the suspension resonance, and becomes a steering rotational vibration through the steering system.
The apparatus disclosed in Patent Document 2 does not entirely consider the brake judder and the shimmy. Further, in Japanese Patent Application Laid-open No. 2002-145075 (Patent Document 3) and Japanese Patent Application Laid-open No. 2002-161969 (Patent Document 4), although apparatuses that damp vibrations of the brake judder and the shimmy are disclosed, both of which are mechanical handling, and there is a problem that the cost increases occurs and a finely-tuned suppression such as the velocity sensitive is not possible.